1. Field of the Invention
The present invention relates to a method and a drive circuit for driving a piezoelectric element, and a liquid-droplet ejection head. More specifically, the present invention relates to a liquid-droplet ejection head, and a method and a drive circuit for driving the liquid-droplet ejection head configured to apply a drive voltage to a piezoelectric element to eject a liquid droplet.
2. Description of the Related Art
There is conventionally known a liquid-droplet ejection head having a piezoelectric element that is mechanically distorted by a voltage application. In general, such a liquid-droplet ejection head is configured to apply a drive voltage to the piezoelectric element to apply a pressure to liquid in a pressure chamber by changing a volume of the pressure chamber filled with the liquid such as ink, thereby ejecting a liquid droplet from a distal end of a nozzle communicating with the pressure chamber.
The piezoelectric element has an electrostatic capacity like a condenser. When an interelectrode voltage of the piezoelectric element (an amount of electric charge charged to the piezoelectric element) is abruptly changed, the liquid may be ejected erroneously from the nozzle. For example, when a drive voltage is suddenly applied to a piezoelectric element whose interelectrode voltage has been lowered by self-discharge, an internal pressure of the pressure chamber changes considerably by the sudden change of the interelectrode voltage of the piezoelectric element, which may cause the misejection of the liquid droplet from the nozzle.
In order to solve this problem, one conventional technique is designed such that the piezoelectric element is gradually charged and discharged upon electric charge and discharge of the piezoelectric element to prevent the sudden change of the interelectrode voltage of the piezoelectric element. FIG. 15 shows one conventional example of a timing chart of a source voltage and drive signals upon the electric charge and discharge of the piezoelectric element. As shown in FIG. 15, upon the electric charge of the piezoelectric element, a driver source voltage (i.e., a HVDD voltage) is changed from a ground level to a reference voltage level over several tens ms in a state in which the driver is set so as to apply the drive voltage to the piezoelectric element (hereinafter may be referred to as “driver ON state”), thereby gradually increasing the interelectrode voltage of the piezoelectric element. Further, as shown in FIG. 15, upon the electric discharge of the piezoelectric element, the driver source voltage is changed from the reference voltage level to the ground level over several hundreds ms in the driver ON state, thereby gradually reducing the interelectrode voltage of the piezoelectric element.
In one conventional technique, in order to prevent the sudden change of the interelectrode voltage of the piezoelectric element upon the electric charge and discharge, one electrode of the piezoelectric element is connected to a power source of the reference voltage level at several times upon the electric charge. That is, the voltage is intermittently applied to the electrode of the piezoelectric element to charge the one electrode of the piezoelectric element stepwise. Upon the electric discharge, the one electrode of the piezoelectric element is connected to a ground at several times. That is, the one electrode of the piezoelectric element is intermittently grounded to discharge electric charge accumulated in the piezoelectric element stepwise.